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Topic 9Maps

"He's off the map!"-Stan (Mark Ruffalo) Eternal Sunshine of the Spotless Mind

Data Structures More than arrays and lists Write a program to count the frequency of all

the words in a file. Make a simplification: assume words are

anything set off by whitespace

Maps 2CS 314

Performance using ArrayList

Maps 3

Title Size(kb)

Total Words

DistinctWords

Time(sec)

small sample 0.6 89 25 0.0012BR02B 34 5,638 1,975 0.051Alice in Wonderland

120 29,460 6,017 0.741

Adventures of Sherlock Holmes

581 107,533 15,213 4.144

2008 CIA Factbook

10,030 1,330,100 74,042 173.000

CS 314

Order? Express change in size as factor of previous file

Maps 4

Title Size Total Words

DistinctWords

Time

small sample 0.6 89 25 0.0012BR02B 57x 63x 79x 51xAlice in Wonderland

3.5x 5.2x 3.0x 14.5x

Adventures of Sherlock Holmes

4.8x 3.7x 2.5x 6.0x

2008 CIA Factbook

17x 12.3x 5x 42x

O(Total Words * Distinct Words) ??

CS 314

Clicker Question Given 3 minutes for the 2008 CIA Factbook

with 1,330,100 total words and 74,042 distinct words, how long for 1,000x total words and 100x distinct words?

A.an hour

B.a day

C.a week

D.a month

E.half a year

Maps 5CS 314

Why So Slow Write a contains method for an array based list

public boolean contains(E target) {

Maps 6CS 314

CS 314 Maps 7

A Faster Way - Maps Also known as:

– table, search table, dictionary, associative array, or associative container

A data structure optimized for a very specific kind of search / access

In a map we access by asking "give me the value associated with this key."

Recall, in the ArrayList example we did not count the number of occurrences of each word

CS 314 Maps 8

Keys and Values Dictionary Analogy:

– The key in a dictionary is a word: foo

– The value in a dictionary is the definition: First on the standard list of metasyntactic variables used in syntax examples

A key and its associated value form a pair that is stored in a map

To retrieve a value the key for that value must be supplied– A List can be viewed as a Map with integer keys

CS 314 Maps 9

More on Keys and Values Keys must be unique, meaning a given key

can only represent one value– but one value may be represented by multiple

keys– like synonyms in the dictionary.

Example:factor: n.See coefficient of X

– factor is a key associated with the same value (definition) as the key coefficient of X

CS 314 Maps 10

The Map<K, V> Interface in Java

void clear()– Removes all mappings from this map (optional operation).

boolean containsKey(Object key) – Returns true if this map contains a mapping for the

specified key.

boolean containsValue(Object value) – Returns true if this map maps one or more keys to the

specified value.

Set<K> keySet() – Returns a Set view of the keys contained in this map.

CS 314 Maps 11

The Map Interface Continued V get(Object key)

– Returns the value to which this map maps the specified key.

boolean isEmpty() – Returns true if this map contains no key-value

mappings.

V put(K key, V value) – Associates the specified value with the specified

key in this map

CS 314 Maps 12

The Map Interface Continued V remove(Object key)

– Removes the mapping for this key from this map if it is present

int size() – Returns the number of key-value mappings in

this map.

Collection<V> values() – Returns a collection view of the values contained

in this map.

Results with Map

Maps 13

Title Size(kb)

Total Words

DistinctWords

TimeList

TimeMap

small sample 0.6 89 25 0.001 0.0008

2BR02B 34 5,638 1,975 0.051 0.0140

Alice in Wonderland

120 29,460 6,017 0.741 0.0720

Adventures of Sherlock Holmes

581 107,533 15,213 4.144 0.2500

2008 CIA Factbook 10,030 1,330,100 74,042 173.000 4.0000

CS 314

Order?

Maps 14

Title Size Total Words

DistinctWords

TimeList

TimeMap

small sample 0.6 89 25 0.001 0.00082BR02B 57x 63x 79x 51x 18xAlice in Wonderland

3.5x 5.2x 3.0x 14.5x 5x

Adventures of Sherlock Holmes

4.8x 3.7x 2.5x 5.6x 3.5x

2008 CIA Factbook

17x 12.3x 5x 42x 16xO(Total Words)?

CS 314